1. Field of the Invention
The present invention relates to a fuel evaporative-gas emission preventing apparatus, and particularly to a fuel evaporative-gas emission preventing apparatus suitable for use in a fuel tank mounted in a vehicle such as a motor vehicle or the like.
2. Description of the Related Art
As a fuel tank mounted in a vehicle such a motor vehicle or the like, there has heretofore been known one equipped with a fuel evaporative-gas emission preventing apparatus which allows a canister to adsorb a fuel evaporative-gas (vapor) generated within the fuel tank. As one example, there is known one described in Japanese Utility Model Application Laid-Open No. 55-15376.
As shown in FIG. 13, this type of fuel evaporative-gas emission preventing apparatus has a breather 74 for coupling the inside of a fuel tank 70 and an opening or port 72A of an inlet pipe 72 to one another. Upon fueling, some of a fuel evaporative-gas in the fuel tank 70 is circulated from the breather 74 to the port 72A of the inlet pipe 72 by using an increase in internal pressure of the fuel tank 70 to thereby reduce the quantity of intake of fresh air from the outside of the inlet pipe 72 to the port 72A, whereby the quantity of generation of vapor can be restrained. As a result, the total quantity of fuel evaporative-gases generated until the time when the fuel tank 70 is fully provided with fuel, can be reduced and a size reduction of a canister 78 connected to an upper portion of the fuel tank 70 by a vapor line 78 can be achieved.
In the fuel evaporative-gas emission preventing apparatus, a purge flow-rate control valve 90 for varying a flow-passage sectional area according to the output of an atmospheric pressure sensor 88 is provided in the course of a line 80 for coupling the canister 78 and a purge port 88 provided on the upstream side of a throttle valve 84 of a carburetor 82 to one another. Therefore, when a sufficient purge flow rate can be ensured even when the atmospheric pressure is reduced. Thus, a canister having a capacity substantially identical to that of a canister used at a flatland can be used even at a highland. The purge flow-rate control valve 90 is coupled to an intake manifold 98 by an intake negative-pressure Supply line 92 through a check valve 94 and an orifice 96 disposed in a direction of preventing a negative pressure on the purge flow-rate control valve 90 side from reduction.
However, when the fuel tank 70 is fueled at a low speed (e.g., 15 liter/min as a typical value) corresponding to a normal fueling speed or a high speed (e.g., 38 liter/min as a typical value) in this type of fuel evaporative-gas emission preventing apparatus, variations in tank internal pressure relative to a fueling time are shown in FIG. 14. Namely, when the fuel tank 70 is fueled at the high speed (38 liter/min), the internal pressure of the fuel tank 70 at the time that the breather 74 is provided, becomes low as compared with the case where no breather 74 is provided. Further, when the breather 74 is provided, the internal pressure of the fuel tank 70 becomes low if the diameter .PHI. of the breather 74 is increased. This is because the quantity of the fuel evaporative-gas circulated in the breather 74 increases, the quantity of intake of fresh air from the outside of the inlet pipe 72 to the port 72A is reduced and no new fuel evaporative-gas is generated within the fuel tank 70. Namely, if the fueling speeds are of the same, then the total quantity of air of intake from the port 72A of the inlet pipe 72 is identical in either case. Some percent of the total quantity of intake air becomes a fuel evaporative-gas in place of fresh air owing to the provision of the breather 74. Since the high fueling speed (38 liter/min) increases the quantity of intake of fresh air as compared with the low fueling speed (15 liter/min) when the fueling speed is changed under the same breather diameter (.PHI. 3 mm), the quantity of a fuel evaporative-gas generated in the fuel tank 70 increases to raise the pressure in the fuel tank 70.
If the fueling speed is of the low speed (15 liter/min) when the breather diameter .PHI. is 3 mm, then the quantity of a fuel evaporative-gas generated by the intake of fresh air becomes 0 g/liter as shown in FIG. 15. Thus, since the intake of fresh air is substantially not produced, no new fuel evaporative-gas is generated. Further, the difference between the quantity of a fuel evaporative-gas generated by the intake of fresh air in the absence of the breather (when the breather diameter .PHI. is 0 mm) and the quantity of the fuel evaporative-gas generated (0 g/liter), a so-called breather effect becomes 0.11 g/liter (=0.11-0). On the other hand, when the fueling speed is of the high speed (38 liter/min), the quantity of a generated fuel evaporative-gas is 0.4 g/liter and a breather effect becomes 0.14 g/liter (0.54-0.4).
Thus, the breather effect can be obtained even if the fueling speed is changed. However, when the diameter of the breather is small, the quantity of the generated fuel evaporative-gas increases on the high fueling speed side. It is thus necessary to increase the capacity of the canister 78.
If the fueling speed is of the high speed (38 liter/min) when the breather diameter .PHI. is 6 mm, then the quantity of the generated fuel evaporative-gas is 0.14 g/liter and the breather effect becomes a large value, i.e., 0.4 g/liter (=0.54-0.14). On the other hand, when the fueling speed is less than or equal to 29 liter/min, the quantity of the generated fuel evaporative-gas becomes 0 g/liter or less. This shows that a vapor leak from the port 72A of the inlet pipe 72 occurs due to an excess in the quantity of the fuel evaporative-gas. Thus, the breather effect becomes great as the diameter of the breather increases but the vapor leak takes place on the low fueling speed side. The vapor leak simply occurs during a minimal time from the commencement of fueling to the completion of fueling, whereas the vapor leak is brought to about 0 when fueling.
When the number of breathers is one even in the case of any breather diameters as described above, the optimum quantity of fuel evaporative-gas, i.e., the quantity of intake of fresh air cannot be achieved to about 0 at all normal gasoline feed speeds.
When the fueling speed is of a high speed (38 liter/min), an increase in breather diameter allows a reduction in the quantity of adsorption of a canister as shown in FIG. 16. This is similar even when the fueling speed is of a low speed (15 liter/min).
However, when the fueling speed is of a high speed (38 liter/min), a variation in emission is small even if the breather diameter is increased, as shown in FIG. 17. On the other hand, when the fueling speed is of a low speed (15 liter/min), an emission exceeds a controlled or restricted value if the breather diameter .PHI. exceeds about 4 mm. Namely, when the breather diameter is small, the quantity of adsorption of the canister is increased so that the capacity of the canister becomes large.
Data about graphs illustrated in FIGS. 14 through 17 show data tested in conformance with the conditions of The Environmental Protection Agency under a fuel-tank capacity of 70 liters, a room temperature of 26.7.degree. C., a tank temperature of 26.7.degree. C. and a supply fuel temperature of 19.4.degree. C.
A problem arises that since the required breather diameter varies according to the fueling speed in this way, the single breather is insufficient to reduce the quantity of adsorption of a canister as low as possible at any fueling speeds and to allow the emission to fall within the restricted value.
As techniques related to the present invention, there have been known a technique described in Japanese Patent Application Laid-Open No. 82-194938 wherein a liquid seal and a negative pressure for fuel injection enables prevention of fuel vapor from being discharged out of a fueling port, a technique described in Japanese Utility Model Application Laid-Open No. 2-12927 wherein the flow of fuel vapor from a fuel tank to a vent tube permits prevention of the fuel vapor from being ejected out of a fueling port, a technique described in Japanese Utility Model Application Laid-Open No. 3-42428 wherein a liquid seal and a negative pressure for fuel injection permits prevention of fuel vapor from being discharged out of a fueling port, and techniques described in U.S. Pat. Nos. 4,714,172 and 4,135,562.